The invention relates to ice protection, and more particularly relates to ice protection systems for use on aircraft. In its most immediate sense, the invention relates to aircraft ice protection systems of the type in which a semi-rigid skin forms a lifting surface of the wing and in which ice is removed by flexing the skin.
Ice contamination of lifting surfaces (e.g. wings and horizontal tails) is always disadvantageous because it interferes with airflow over the surface. This in turn increases drag, reduces lift, and reduces the angle of attack at which the airfoil enters a stall. For this reason, airplanes are provided with systems that protect lifting surfaces from excessive levels of ice contamination on critical regions of the lifting surface.
Ice protection systems vary widely in performance. This is because different types of aircraft lifting surfaces have different sensitivities to ice contamination and, consequently, different ice protection requirements. For example, certain lifting surfaces may use airfoils that are more or less tolerant to the effects of ice contamination than are other airfoils; a quantity of accreted ice that might only imperceptibly degrade the performance of one type of airfoil section might be a severe hazard to another type of airfoil when operated at similar or different flight conditions.
Accordingly, different types of ice protection systems are optimized differently; energy available for ice protection is selectively applied to address the particular sensitivity of one airfoil section as opposed to another. On some airplanes, there may be sufficient electrical power to operate an ice protection system that relies on electrically-generated heat (e.g., an electro-thermal deicing or anti-icing system) while on others, the available electrical power is insufficient for such a system. Such considerations necessarily affect the selection of the type of ice protection system to be used. For example, on an airplane having ample electrical power or bleed air, an energy intensive evaporative anti-icing system may be employed, while for a power-limited application a deicing system may be used to shed ice only when the ice accumulation reaches a predetermined distributed thickness that has been shown to degrade the performance of the airfoil to an unacceptable extent.
Although many different types of ice protection systems are available to address a wide variety of applications for ice protection, one application category is particularly problematic. This is when the airfoil is very sensitive to ice contamination and there is limited power available for operation of an ice protection system that meets the required performance.
The present invention is suitable for applications of this type. Tests have demonstrated that a preferred embodiment of this invention can maintain worst-case distributed ice accretions to within critical limitations (typically less than 0.050 inch) while consuming only a fraction of the power that would conventionally be expected to be required for such an application.
The invention proceeds from a realization that an existing ice protection system can be reconfigured to operate in an entirely different and highly advantageous manner. Commonly-owned U.S. Pat. No. 5,921,502 (incorporated herein by reference, and referred to hereinafter as the “'502 patent”) discloses a hybrid ice protection system in which an airfoil has a semi-rigid skin 58. The skin 58 is caused to flex by actuators 50, 52, 54, and 56, and the leading edge region 4′ is heated by an electrical heater 10′. In operation, the electrical heater 10′ is operated as a running-wet anti-icer and ice accreted aft of the leading edge region 4′—so-called “runback refreeze” ice—is periodically removed by the actuators 50, 52, 54, and 56. In essence, the '502 patent discloses an ice protection system that uses heat to prevent ice contamination where the airfoil is most roughness-sensitive and uses mechanical flexing of a semi-rigid skin to shed accreted ice from locations where the airfoil is less so.
The inventor of the present invention realized that energy efficiency of such a hybrid system would be much improved if the heat and the flexing of the semi-rigid skin were employed together in a coordinated fashion instead being used independently in different locations. In accordance with the invention, heat and mechanical deformation are both applied to a region to be protected, but the heat is used only to increase the temperature of the ice/skin interface, reducing the adhesion force between the ice layer and the subjacent skin and thereby weakening the bond between the skin and the ice that has accreted upon it instead of removing the ice by melting it into water (as in the prior art). Once this bond has been weakened, two things happen approximately simultaneously: the heat is turned off, and the actuators are fired to flex the skin. Because the bond between the ice and the skin has been substantially reduced or eliminated, the flexing of the semi-rigid skin completely sheds the accreted ice. Furthermore, the absence of heating between deicing cycles causes the temperature of the skin to drop below freezing before runback refreeze can be created. Consequently, much less overall heat is delivered to the protected region, and this greatly improves the energy efficiency of the system.
Advantageously although not necessarily, and in the preferred embodiment, energy consumption of the system is further reduced by removing accreted ice from the airfoil on a zone-by-zone basis. This is accomplished by dividing the airfoil into a series of zones that extend along the span of the airfoil and then applying heat to individual zones sequentially, one at a time. This avoids the power drain that would be required to heat the entire airfoil at once. In particular, it is possible to eliminate the continuously heated parting strip along the entire span of a typical electro-thermal de-icing system.
In the preferred embodiment, heating continues until a very thin layer of ice at the ice/skin interface is melted immediately adjacent the skin. This insures that the accreted ice is completely shed when the skin is deformed. However, this is only preferred, and it may be possible to obtain acceptable performance even if the ice is not entirely melted at the surface of the skin.
In the preferred embodiment, the heating and flexing are co-located in the region of the airfoil that is most sensitive to the effects of accreted ice. This applies the maximum heat and the maximum mechanical force to the accreted ice in the location where ice will most seriously degrade aerodynamic performance. However, it will be understood that even if the ice to be removed is at some distance from the heater, the actuators, or both, it is nonetheless possible that the ice can be removed. Furthermore, in accordance with the preferred embodiment, regions of the airfoil that are less sensitive to ice contamination are protected only by flexing of the skin, as in the '502 patent.
In the preferred embodiment, heating is electro-thermal. However, this is only preferred, and it may be possible to obtain acceptable performance using bleed air as a source of heat.